1. Field of the Invention
This invention relates to a paper exhaustion detector for a paper feeder adapted to feed paper with a contact portion of an actuator of the exhaustion detector contacting the lower surface of the paper, and detect the exhaustion of the paper when the contact portion of the actuator is displaced to a position higher than a set position.
2. Prior Art
A paper feeder is installed in a printer for a computer and a word processor and for used to print words on paper 12 which has perforations 12a in both edge portions thereof as shown in FIG. 2, and which is fed intermittently.
The paper 12 having such perforations 12a is also called continuous paper but even such continuous paper necessarily runs out during printing.
If a printing operation is continued with the paper exhausted, the head of a printer works idly to print letters uselessly on a platen (a member holding the paper, disposed on the opposite side of the head of a printer and receiving impact during a printing operation), so that both the head and platen are damaged.
To prevent these inconveniences, it is necessary that a paper exhaustion detector be provided at the portion of a path of paper which is immediately before the head of a printer, the paper exhaustion detector being attached unitarily to a paper feeder.
A conventional paper feeder provided with such a paper exhaustion detector is shown in FIGS. 26-30.
An endless feed belt 8 for feeding paper 12 is provided between first and second side frames 2a, 2b, and has a plurality of feed pins 9 adapted to be inserted into the perforations 12a in the paper 12 and extending outwardly at predetermined intervals.
A cover 13 for preventing the paper 12 being fed from floating out of engagement with the feed pine 9 is joined to the first side frame 2a so that the cover 13 can be opened and closed, and a microswitch 26 is attached to the second side frame 2b.
The second side frame 2b is provided with a paper receiving plate 24 extending sideways and horizontally therefrom and formed integrally therewith, and the microswitch 26 is attached to the lower side of this paper receiving plate 24.
An actuator 27 is provided between the paper receiving plate 24 and microswitch 26, which is operated by this actuator 27. The actuator 27 is formed so that it can be turned around a fulcrum pin 28, and a contact portion 27a of the actuator 27 projects above the upper surface of the paper receiving plate 24 through a bore 29 provided in the same plate 24.
When the cover 13 joined to the first side frame 2a is closed, it extends over the paper receiving plate 24. Guide ribs 14 project in parallel with the side frames 2a, 2b from the portions of the rear surface of the cover 13 which are opposed to the first and second side frames 2a, 2b when the cover 13 is in a closed position. These guide ribs 14 prevent both edge portions of the paper 12 from floating.
A pin plunger 31 of the microswitch 26 is urged in its projecting direction by a compression spring (not shown) contained in the microswitch 26, and contacts the lower surface of a base end portion of the actuator 27. Therefore, the actuator 27 receives from the pin plunger 31 first angular moment M.sub.1 by which the contact portion 27a thereof is displaced upward, and also from the paper 12 passing between the paper receiving plate 24 and cover 13a second angular moment M.sub.2 by which the contact portion 27a is displaced downward. The position of the contact portion 27a of the actuator 27 is fixed when the first and second angular moments M.sub.1, M.sub.2 act against which the actuator 27 in opposing directions, are balanced.
FIG. 28 shows a condition of the actuator 27 when the second angular moment M.sub.2 largely exceeds the first angular moment M.sub.1 and causes the upper surface of the contact portion 27a of the actuator 27 and that of the paper receiving plate 24 to become substantially flush with each other.
In this condition, the projecting length of the pin plunger 31 is D.sub.1, and a signal representative of the presence of the paper 12 between the paper receiving plate 24 and cover 13 is outputted from the microswitch 26.
FIG. 29 shows a condition of the actuator 27 when the first and second angular moments M.sub.1, M.sub.2 are balanced, with a distance a between the upper surface of the contact portion 27a of the actuator 27 and that of the paper receiving plate 24 which is larger than a distance b between the upper surface of the paper receiving plate 24 and the lower surface of the opposed guide rib 14. In this condition, the projecting length of the pin plunger 31 is D.sub.2, which indicates a limit level up to which a signal representative of the presence of the paper 12 is outputted from the microswitch 26. Namely, even when the contact portion 27a of the actuator 27 is displaced toward the cover 13 by a distance a above the lower surface of the guide rib 14, a signal representative of the presence of the paper is outputted.
FIG. 30 shows a condition of the actuator 27, where the first angular moment M.sub.1 alone works on the actuator 27 to cause the contact portion 27a thereof to contact the rear surface of the cover 13. In this condition, the projecting length of the pin plunger 31 is D.sub.3, and a signal representative of the exhaustion of the paper (i.e. absence of the paper) is outputted from the microswitch 26. When a paper exhaustion signal is outputted, the rotation of a driving shaft 32 for driving the feed belt 8 is stopped, and the printing operation 19 of the head of the printer is also stopped simultaneously, so that an idle printing operation of the head of the printer is prevented.
The projecting lengths D.sub.1, D.sub.2, D.sub.3 of the pin plunger 31 naturally have the following relation. EQU D.sub.1 &lt;D.sub.2 &lt;D.sub.3
The contact portion 27a of the actuator 27 contacts the lower surface of the paper 12 passing between the paper receiving plate 24 and cover 13, and the portion of the paper 12 which passes through this narrow clearance presses down the contact portion 27a of the actuator 27 to cause the second angular moment M.sub.2 to be exerted on the actuator 27.
The level of the pressing force of the paper 12 applied to the contact portion 27a of the actuator 27 is determined by the tensile force applied to the paper 12 and the rigidity (hardness or firmness) of the paper 12 itself. The position of the contact portion 27a of the actuator 27 is determined when the second angular moment M.sub.2 occurring due to the pressing force of the paper 12 applied to the contact portion 27a and the first angular moment M.sub.1 occurring due to the projecting force of the pin plunger 31 of the microswitch 26 are balanced. When the distance between the upper surface of the contact portion 27a and that of the paper receiving plate 24 is kept smaller than the level a, a signal representative of the presence of the paper is outputted.
However, when the paper 12 being fed is loosened and has a low tensile strength, or when the paper 12 being fed has a low rigidity due to its small thickness or high flexibility, the level of the pressing force of this paper 12 applied to the contact portion 27a of the actuator 27 is small.
In such cases, the paper 12 is deformed due to the contact pressure of the actuator 27 pressing upwardly as shown in FIGS. 26 and 27, and the first and second angular moments M.sub.1, M.sub.2 are balanced, with the distance a' between the upper surface of the contact portion 27a of the actuator 27 and the upper surface of the paper receiving plate 24 larger than the distance a referred to above. Consequently, a paper exhaustion signal is outputted in spite of the presence of the paper 12 between the paper receiving plate 24 and cover 13.
In some cases, when the paper being fed is suddenly stopped, the contact portion 27a of the actuator 27 is not positioned properly and a paper exhaustion signal is erroneously generated.
When the detection of exhaustion of paper is thus made erroneously, the printer is stopped erroneously, so that the printing operation is interrupted to cause the paper 12 which had already been printed upon to be rendered unusable, and the user incurs a great loss due to the unnecessary interruption of the printing operation.